A Retrospective Study of Bowel and Rectum Air Effect on Dose Coverage in Prostate, Colon, Gynecologic and Embryonal Rhabdomyosarcoma Tumors Treated With Robust Intensity-Modulated Proton Therapy

Abstract

For pelvis patients treated with intensity-modulated proton therapy (IMPT), robust treatment plans were made by using 3 CT sets: the planning CT (pCT) set and 2 virtual CT sets that were copies of pCT but where the fillings of bowels and rectum were overridden to be either air or muscle. The hypothesis is that assessment on the air volume during patient setup is still necessary for IMPT with such robust plans. The endpoints are the mean change of CTV dose D99 and mean CTV dose D99. The assessment is assumed to be necessary if the mean change of D99 of the CTV >3% and the mean of D99 of the CTV <95%. Data from 17 cancer patients (11 prostate, 3 gynecologic, 2 colon, and 1 embryonal rhabdomyosarcoma) with pCT and weekly or biweekly scanned quality assurance CTs (QACTs; 82 QACT scans total) were studied. Air in bowels and rectum traversed by proton pencil beams was contoured. The targets in the pCT were mapped to the QACTs by deformable image registration, and the dose in QACTs was calculated with original beamline data. Dose coverage (D99 and D95) and correlations between dose coverage and changes in air volume were analyzed. The significance of the correlation was analyzed by t test. For a majority of the treatment course, the air volume during treatment was larger than that during pCT scanning. The mean (stdev) change of air volume in beam path was 0.1(0.3), 2.4(1.2) and 11.2(3.6) cc for the air in rectum, small- and large-bowels respectively. For the relative change of dose, the mean changes of D99 in QACTs were within 3% of those in the pCT for all prostate and colon cases but >5% in 2 of the 3 gynecologic cases and >3% in the embryonal rhabdomyosarcoma case. Of these 3 cases with mean change of D99 >3%, air volume was the main cause in 2, where the locations of air in the pCT and some QACTs were different, and the air was on the path of multiple beams. These resulted in accumulated dose deviation. Mean changes of D95 were systematically less than those of D99, and only in one gynecologic case the change was >3%. In the QACTs of the boost plans for those prostate bed cases, the air in bowels and rectum was not in the lateral beam paths, and the mean change of D99 andD95 was <1.5% and <0.5% respectively. Correlations were of medium strength between change in air volume and change in D99 and D95, because other anatomic changes such as patient body shape, weight, bladder filling, and femoral bone rotation also contributed to dose deviation. The correlation was statistically significant. For the absolute change of dose, increases in air volume change reduced target dose coverage in QACTs. In these 17 cases, only 2 had CTV D99 < 95% of prescription dose (Dx) and D95 < 95% of Dx in the original plan, but 5 had means of D99 < 95% of Dx and 2 had D95 < 95% of Dx in QACTs. The effect of air volume on target dose coverage is significant in treatment plans using 3 CT sets, particularly when the air is traversed by multiple beams. We suggest contouring the air and beam path in pCT and using the contours to assess patient preparation and setup.